In Undead Horde you'll level up as in many games with RPG elements. When leveling up you earn points which you can spend on three different subclasses.
Here's our current level up screen. It still needs improvement and polish, but it should give you the basic idea.
Here you can spend points on various subclasses. Each choice improves the necromancer attributes a bit differently. The choice depends on your desired playstyle.
If you like to beat the hell out of the enemies the Executioner helps with that. If you'd like to have a large horde, you should spend points on Re-Animator. If you like to cast spells a lot, then spend points on Arcanist.
You can also mix and match as you will - however some items may have subclass rank requirements so you may need to rank up certain subclass to use them.
Here's an example of rank 10 arcanist attributes. Notice the large manapool.
The heart icon represents hit points, the mana potion is mana, the banner is command and the orange impact icon is the damage bonus. Your necromancer attributes will be the sum of the choices you've made along the way.
Arcanist focused Necromancer wreaking havoc with fireballs. With focus on Arcanist the larger mana pool allows extensive spell attacks.
Rank 10 Executioner. The damage bonus is +66% and the hit points are very high.
A necromancer with focus on Executioner showing of his signature weapon - the Scythe. Even after his horde is gone, he continues to fight since he has some hitpoints to spare.
The Scythe can be very effective as it hits multiple enemies. You need to spend some points on the Executioner subclass to use this weapon.
Rank 10 Re-Animator. Command cap at 41 is much higher than the others.
In this final image a Re-Animator focused necromancer commands his massive undead archer horde to attack and buffs them with flaming arrows.